Saying "Garrosh cheated" carries the implication that he knew of what was happening beforehand - Magatha was the only person who had that knowledge, so she could really be the only person to actively cheat. The Mak'gora in this case was not a legitimate victory for Garrosh is all that can really be said about Garrosh's role in it - had Magatha not intervened it would've likely been the end of Garrosh as well, as Cairne seemed to be winning the competition pretty handily until the poisoning kicked in. Garrosh was painfully and humiliatingly aware that Magatha's actions also cost him the moral or honorable high ground, as his letter to Magatha is more about his own wounded pride than the loss of Cairne as a valued leader of the Horde.
"We're more of the love, blood, and rhetoric school. Well, we can do you blood and love without the rhetoric, and we can do you blood and rhetoric without the love, and we can do you all three concurrent or consecutive. But we can't give you love and rhetoric without the blood. Blood is compulsory. They're all blood, you see." ― Tom Stoppard, Rosencrantz and Guildenstern are Dead
This is not about implications or "actively cheating", it's about the mere act of cheating. Magatha couldn't "cheat" anything as she wasn't Cairne's adversary, Garrosh was. The fact that she desired a certain outcome and rigged the duel is not the same as actively participating in the competition and carrying the act of cheating. The only one who can "cheat" is the one who competes. Mind you, I see from where you come from but on the end of the day the expression feels wrong to you because it sounds in your minds through the most common situation involving cheating (aka the challenger being aware, crooked or the sole driving force behind the dishonest act) rather than through the most basic meaning of the word.
I basically see it as an outcome engineered by a third-party, essentially - like a football game where some outside party conspired with the referees to penalize one team while simultaneously giving the other team a pass on possible fouls. It's not the fault of the winning team that this occurred if they had no knowledge of it, and it wouldn't be fair to accuse them directly of cheating if they weren't aware they were the beneficiaries of this third party's actions. You could blame the referees for cheating, of course; and in this case Magatha was more or less the party acting in bad faith where the expectation would be otherwise. It is odd to say Magatha cheated without her being directly involved in the duel - but I guess the charge would be that she manipulated the outcome while both Garrosh and Cairne were otherwise unaware. It would be different if Garrosh had been aware (or become aware during the duel) of Cairne's poisoning - just like with football analogy if the other team became aware of their preferential treatment and said nothing then you could make a case for them being complicit (and thus actively cheating).
"We're more of the love, blood, and rhetoric school. Well, we can do you blood and love without the rhetoric, and we can do you blood and rhetoric without the love, and we can do you all three concurrent or consecutive. But we can't give you love and rhetoric without the blood. Blood is compulsory. They're all blood, you see." ― Tom Stoppard, Rosencrantz and Guildenstern are Dead
One man's villain is another man's hero. Personally, since he exemplified what the Warchief of the Orcish Horde should be like, I loved his character. However, by the end, I was ready for him to go - albeit, not with Thrall stealing the kill.